1. Field of the Invention:
The present invention relates generally to a solid state image sensing device, and is directed more particularly to a solid state image sensor device of a frame transfer system which employs a charge transfer device (which will be referred hereinafter to as a CTD).
2. Description of the Prior Art:
A prior art solid state image sensing device of the frame transfer system will be described with reference to FIG. 1. This prior art image sensor device consists of an image sensing array 1 which will produce an electric charge or carrier pattern corresponding to picked-up pattern or amount of received light, a temporary storage array 2 which temporarily stores the charge pattern from the image sensing array 1, and a shift register 3 which sequentially transfers the signals from the storage array 2 to an output terminal t. The image sensing array 1 is formed of a plurality of CTDs 4 aligned in the vertical direction, the temporary storage array 2 is formed of a plurality of CCDs 4 aligned in correspondence with the arrays 4 of the image sensing array 1, and the shift register 3 is formed also of the CCD 4. In order to produce an electric charge or carrier pattern corresponding to a light image to be received on the image sensing array 1, image sensing cells are formed in the image sensing array 1.
An example of the image sensing array 1 will be now described with reference to FIGS. 2 and 3. A semiconductor substrate, for example, a silicon substrate 5, which is of low impurity concentration, has provided on its one major surface an insulating layer 6 made of silicon dioxide SiO.sub.2 or the like. On the insulating layer 6 there are formed strip electrodes 7 which are arranged common to the respective CTDs 4, and extended in the horizontal direction with a gap G between adjacent ones. Every third electrode 7 is connected together and the electrodes 7 connected together are supplied with 3-phase clock pulses .phi..sub.1, .phi..sub.2 and .phi..sub.3, respectively. Between adjacent CTDs 4 there is formed a strip channel stopper region 8 of high impurity concentration which faces the major surface of the semiconductor substrate 5.
An image sensing cell 9 is formed in the gap G between the adjacent electrodes 7 and between the adjacent channel stoppers 8. Electric charges or carriers produced on the image sensing cells 9 in response to the amount of received lights are transferred successively in, for example, the column direction by the clock pulses .phi..sub.1, .phi..sub.2 and .phi..sub.3, and then transferred to the temporary storage array 2.
With the prior art solid state image sensing device constructed as above, since the image sensing cell 9 is formed between the adjacent transfer electrodes 7, it is desired so as to increase its light receiving efficiency that the width of the gap G is selected large. However, if this width of the gap G is made large, a disadvantage results from the fact that the carrier transfer efficiency is lowered; that is, the light receiving efficiency is contradictory to the carrier transfer efficiency.
FIG. 4 shows another example of the CTD 4 which forms the image sensing cell 9. In the example of FIG. 4, an impurity is selectively doped at high concentration to the insulating layer 6 formed on the semiconductor substrate 5 to form first strip transfer electrodes 7A, which are made of a polycrystalline silicon layer of low resistivity and which are arranged with a predetermined gap between adjacent ones. Then, a second insulating layer 6' made of SiO.sub.2 is coated so as to cover all of the strip electrodes 7A and the insulating layer 6 between the adjacent electrodes 7A, and a second strip transfer electrode 7B made of, for example, aluminum is formed on the portion between the first electrodes 7A on which the insulating layers 6 and 6' are formed. The first and second electrodes 7A and 7B are electrically connected at their one ends to form the transfer electrode 7. The every second transfer electrode 7 is connected commonly to provide two sets of electrodes. The two sets of electrodes 7 are supplied with the clock pulses .phi..sub.1 and .phi..sub.2 to transfer the carriers. Such a type of CTD is called a 2-phase CTD In such a type of CTD, the image sensing cell 9 is formed between the adjacent second electrodes 7B of the transfer electrodes 7 and receives light along arrows a in FIG. 4.
In the case that the above construction of CTD is used, there is produced no gap between the electrodes 7 on the surface of the substrate 5 in the carrier transfer direction, so that the carrier transfer efficiency can be improved relatively. However, since polycrystalline silicon layer or first electrode 7A exists in the image sensing cell 9, the light reception is carried out through the polycrystalline silicon layer 7A. As a result, there occurs such a disadvantage that its sensitivity for the light, especially the light of short wave is lowered.